1. Field of the Invention
This invention relates to granular activated carbon manufacture, and more particularly to a new and improved process for making granular activated carbon from brown coal treated with dilute inorganic acid, and to a new and improved granular activated carbon made by such process and having properties which make it suitable for use in water and waste water treatment and in other applications.
2. Glossary of Terms
In order to facilitate a clear understanding of this invention, various terms of art employed herein are defined as follows.
Abrasion number--is a pressure of the resistance of the activated carbon granules to degrading on being mechanically abraded. It is measured by contacting a sample with steel balls in a pan on a machine and shaking the contents for a given time and determining the resultant particle size distribution and hence the mean particle diameter. The abrasion number is the ratio of the final average (mean) particle diameter to the original average (mean) particle diameter (determined by screen analysis) times 100.
Activated carbon--is a carbon which is "activated" by heating to high temperature preferably with steam or carbon dioxide as the gaseous activating agent in producing an internal porous particle structure.
Activating--means heating coal at high temperatures on the order of about 600.degree. C. to about 1000.degree. C. in the presence of a gaseous activating agent, as is well known in the art.
Adsorption isotherm--is a measurement of the adsorptive capacity of an absorbent (viz. granular activated carbon) as a function of the concentration, or pressure, of the adsorbate (viz. N.sub.2) at a given temperature. It is defined as the constant temperature relationship between the amount adsorbed per unit weight of adsorbent and the equilibrium concentration, or partial pressure.
Apparent density--is the weight per unit volume of homogeneous granular activated carbon. To assure uniform packing of the granules during measurement, a vibrating trough is used to fill the measuring device.
Ash--is a principal mineral constituent of coal, carbon and pitch. It is normally defined as a weight percent basis after a given amount of sample is reduced to ash.
Average (mean) particle diameter--is a weighted average diameter of a granular activated carbon sample. A screen analysis is run and the average particle diameter is calculated by multiplying the weight of each fraction by its average diameter, adding the products, and dividing by the total weight of the sample. The average diameter of each fraction is taken as the size midway between the sieve opening through which the fraction has passed and the sieve opening on which the fraction was retained.
Brown coal--is a low rank lignitic type coal which is ranked below sub-bituminous coal, together with and usually slightly below lignite. Typically, in the moist, as received, or as mined condition, it has a moisture content of more than 40% by weight and generally a moist BTU/lb. of less than 8300, and usually is unconsolidated. See ASTM standard D-388-66, wherein BTU wise, "brown coal", as used herein, would be included in the designation "Lignite A" or "Lignite B", but also see former ASTM standard D-388-58, wherein "brown coal", as used herein, is distinguished from and ranked below "lignite", as used herein, by reason of brown coal's usual unconsolidated characteristic.
Charring--means heating coal at low temperatures on the order of about 175.degree. C. to about 275.degree. C. in the presence of oxygen.
Coking value--is usually expressed as percent residual carbon obtained when a dry sample of coal, tar or pitch is vaporized or pyrolized for a specific time at a specific temperature that limits the available oxygen supply (ASTM Method D-2416). The coking value, expressed as percent residual carbon, indicates the coke forming properties of the material.
Devolatilizing--means heating coal at intermediate temperatures on the order of about 400.degree. C. to about 600.degree. C. in an oxygen-free atmosphere.
Granular activated carbon--is "activated carbon" which has a particle size, i.e., "mesh", which is not less than about 40 and preferably not less than about 60.
Iodine number--is the milligrams of iodine adsorbed by 1 gram of granular activated carbon at an equilibrium filtrate concentration of 0.02 N iodine. It is measured by contacting a single sample of carbon with an iodine solution and extrapolating to 0.02 N by an assumed isotherm slope. This number can be correlated with the ability of granular activated carbon to adsorb low molecular weight substances.
Lignite--is a low rank lignitic type coal which is ranked below sub-bituminous coal, together with and usually slightly above brown coal. Typically, in the moist, as received, or as mined condition, it has a moisture content of more than 20% by weight and generally a moist BTU/lb. of less than 8,300, and usually is consolidated. See ASTM standard D-388-66, wherein BTU wise "lignite", as used herein, would be included in the designation "Lignite A or "Lignite B", but also see former ASTM standard D-388-58, wherein "lignite", as used herein, is distinguished from and ranked above "brown coal", as used herein, by reason of lignite's usual consolidated characteristics.
Mesh--(or mesh size) is the particle size of granules as determined by the U.S. Sieve Series or the Tyler Series. Usually, this term refers to the sizes of the two screens, in either of the above Series, between which the bulk of a sample falls. For example, "8/30 mesh" (or "8 by 30 mesh" or "8.times.30 mesh") means that 90% by weight of the sample will pass through a No. 8 screen but will be retained on a No. 30 screen. Alternatively, this term refers to a maximum particle size, such as in defining the fineness of powder material. For example, "65% by weight -325 mesh powder" means that 65% by weight of a given sample passes through a No. 325 mesh screen.
Molasses number--is calculated from the ratio of the optical densities of the filtrate of a molasses solution treated with a standard activated carbon and the activated carbon in question.
Pitch--is a black or dark viscous substance obtained as a residue in the distillation of organic materials and especially tars.
Powder--means powdered activated carbon which has a particle size, i.e., "mesh", which is smaller than about 40 and preferably smaller than about 60. The larger the mesh number, the smaller the size.
Surface area--is the amount of surface area per unit weight of granular activated carbon; it is determined from the nitrogen adsorption isotherm by the Brunauer, Emmett and Teller (BET) method, and is is expressed in m.sup.2 /gram.